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发表于 2007-10-4 15:06:20
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Stem cell transplantation
Overview
Bone Marrow Basics
Transplant Fundamentals
Stem Cell Collection
Processing
Therapy and Transplantation
Pancytopenia to Engraftment
Potential Complications
Stem cell transplantation (SCT) is a rapidly advancing treatment option for patients who have cancer or an immunological disorder. Over the past 30 years, transplantation has evolved from an experimental treatment for a small group of diseases to a standard of care for many blood and immunologic disorders and cancers.
Bone marrow and peripheral blood stem cell transplantation is a therapeutic procedure used to treat malignant disorders that have relapsed, become resistant to therapy, or are incurable with standard therapy.
Disease Chart
Stem cell transplantation may also be used as a potentially curative treatment of nonmalignant and/or genetic disorders such as aplastic anemia, severe combined immunodeficiency syndrome (SCIDS), thalessemia, or sickle cell anemia.
Bone Marrow Basics
The bone marrow is the spongy core found in the center of bones and is the source of all stem cells. Stem cells are the precursor cells responsible for the formation of the blood or hematopoietic system (red blood cells, platelets, and white blood cells). The figure demonstrates how blood cells develop from a stem cell.
Stem Cell Tree
Stem cells are capable of self-replication, or forming additional stem cells, and differentiation, committing themselves to the formation of the blood cell lines. Red blood cells, or erythrocytes, carry oxygen to the tissues. Platelets, or thrombocytes, assist with clotting and control bleeding. White blood cells, or leukocytes, help fight infections. White blood cells are further differentiated or classified as neutrophils, monocytes, lymphocytes, basophils, and eosinophils. The bone marrow also supplies the cytokines or growth factors that provide a nutrient environment in which the cells will mature. New blood cells are constantly being produced by the bone marrow on an as needed basis.
Chemotherapy and radiation therapy affect healthy cells as well as tumor cells, particularly fast-growing and dividing cells like those found in the bone marrow. The suppression of the bone marrow function after chemotherapy is the side effect that most often determines and limits the doses of therapy that can be given to and tolerated by a patient.
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Transplant Fundamentals
There are two basic and distinct reasons to perform a bone marrow and/or peripheral blood stem cell transplant:
1. To treat malignancies with high doses of chemotherapy and/or radiation therapy and rescue the bone marrow function or
2. To replace malfunctioning bone marrow with healthy, functioning bone marrow
Transplants are defined as autologous or allogeneic.
Autologous Stem Cell Transplants
In autologous transplants, the recipient serves as his or her own donor. For example, a patient with breast cancer will undergo high-dose chemotherapy followed by an autologous transplant to rescue or replace the bone marrow destroyed by therapy. The diseases commonly treated with an autologous transplant include solid tumors, such as breast cancer and ovarian cancer, non-Hodgkin's lymphoma, Hodgkin's disease, and some of the leukemias.
Cancer Patient
blood drop
Patients are assessed by a transplant team in order to determine eligibility for a stem cell transplant. Considerations include physical health, disease stage, and existence of a strong support network. Once a patient is accepted for transplant, he or she will may receive chemotherapy in order to reduce the amount of tumor in the body.
The chemotherapy may be given in 1-4 cycles over several months. The patient will then repeat tests to make sure that their heart, lung, kidney and liver function is acceptable. The disease stage of the patient will also be repeated to determine how much, if any, tumor remains in the body.
The patient's stem cells will then be collected by apheresis or bone marrow harvest. Stem Cells collected
The stem cells will be processed and cryopreserved.
Stem cells processed Stem cell's Cryopreserved
blood drop
After receiving high dose chemotherapy with or without total body irradiation, High Dose Therapy
Stem Cells Reinfused the patient will receive their own stems cells back through a central venous catheter.
The stem cells will find their way back to the bone marrow to begin their job of making blood cells.
The patient usually engrafts quickly without risk of graft vs. host disease.
Allogeneic Stem Cell Transplants
Stem Cells Collected In allogeneic transplants, a donor provides the stem cells for transplantation to a recipient. For example, a patient with leukemia has malfunctioning bone marrow and would receive an allogeneic transplant of stem cells from a donor following high dose or marrow ablative chemotherapy or radiation therapy. A third type of transplant may be referred to as syngeneic. This is an allogeneic transplant where the donor is the identical twin of the patient. Allogeneic transplants are much more complex than autologous transplants with more potential risk. They are used to treat patients with leukemia, aplastic anemia, lymphoma and immunodeficiency syndromes.
First and foremost, a suitable donor must be found using human leukocyte antigen (HLA) typing. The search is started within the patient's family first, generally siblings, and if necessary, the search continues through international donor registries. A suitable HLA-matched donor is found for only 30-38% of patients in need of an allogeneic transplant. The matched donor is then given a history and physical assessment and the method of stem cell collection is determined.
High Dose Chemotherapy
Stem Cell Transplant Medication given
The stem cells can be cryopreserved or collected on the scheduled day of infusion. The recipient is given the stem cells following high dose therapy.
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Stem Cell Collection
The process in which stem cells are collected provides further definition to the type of transplant received. Stem cells can be collected or harvested directly from the bone marrow. The patient or donor is placed under general anesthesia in an operating room. The transplant physician uses a large bore aspiration needle inserted into the back of the hip bone to aspirate or draw out the bone marrow. Although only one puncture may be needed in each iliac crest, the needle is manipulated and turned frequently until adequate cells have been collected.
Stem cells can be collected from the peripheral blood after the bone marrow has been mobilized with growth factors to produce stem cells and send them in large numbers into the peripheral blood.
Mobilization is accomplished by giving daily injections of granulocyte and/or granulocyte macrophage colony stimulating factor (G-CSF or GM-CSF) with or without moderate doses of chemotherapy. The stem cells increase in numbers and are driven into the blood. They are collected through the process of apheresis which separates the blood into components, draws out the part containing the stem cells and returns the remaining components to the donor. Apheresis is performed over several hours daily until adequate numbers of stem cells are collected. Most patients or donors require the placement of a large central venous catheter to assure a good, stable access for blood processing and return.
The decision of how to collect the stem cells is based on many factors that are assessed and determined by the transplant physician. The goal is to obtain an adequate number of stem cells to restore the bone marrow function following high dose therapy. Then, the stem cells are processed for storage in liquid nitrogen.
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Processing
After stem cells are collected by either apheresis or bone marrow harvest, they are transported to the Stem Cell Transplant Laboratory for processing. There are several different ways in which stem cells can be processed for storage and transplantation. Whichever method is used depends on the needs of the patient and the type of stem cells that are collected.
When peripheral blood stem cells are collected, the cells are processed under sterile conditions by washing with a cold solution (Medium-199®) and centrifuging the container. The volume of the stem cells is reduced by removing the liquid part that does not contain cells. This step can reduce the volume from 300 mI down to 50 mI. Bone marrow stem cells are also processed in this manner. After the washing step, the stem cells are gently mixed in a blood bag and placed in an ice bath. A cold solution that contains donor plasma, heparin, and dimethyl sulfoxide (DMSO) is slowly added to the stem cells. This solution is important for freezing, or cryopreservation of the stem cells. The plasma gives the stem cells a supply of protein during the storage time. The heparin prevents the stem cells from sticking or clumping together. The DMSO is a stem cell protectant for the freezing process and prevents the cells from breaking open during freezing.
Small samples (<5%) of the stem cells are removed during processing in order to count the number of stem cells in the product and test for sterility. Counts are performed using an automated blood counter. The technologists determine how many white blood cells are in each component and the percentage of each subtype of white blood cells including lymphocytes, monocytes, and granulocytes. Mononuclear cells which are lymphocytes and monocytes, are adjusted on the patient's kilogram weight to the number of mononuclear cells/kg. Usually, patients are given 100 million to 1 billion mononuclear cells/kg of weight for a transplant.
Part of the sample is used to specifically identify the true stem cells in a component. Peripheral blood stem cells from a donor who has received growth factor such as Neupogen or G-CSF, usually represent 0.01% to 1% of the white blood cells collected. A bone marrow harvest contains 0.5% to 5% stem cells from the total white blood cells. The method used to identify these cells requires mixing the sample with a fluorescent dye that attached to only the true stem cell. The place of attachment is called cluster of differentiation-34 or CD34. Frequently, stem cells are referred to as CD34 cells. By collecting this sample in a automated machine called a flowcytometer, the technologist can look at 75,000 to 100,000 white blood cells. The cells that fluoresce are true stem cells and are counted by the machine. Using this test, the percentage of stem cells in a product can be calculated. Then, this percentage is applied to the total number of white blood cells in a component. A total number of stem cells is determined based on the kg weight of the patient. Apheresis collections will usually continue until at least 2 million stem cells / kg weight of the patient is reached. This target number is necessary for rapid engraftment, or recovery of cell counts, following high dose chemotherapy.
The final step in processing consists of freezing small samples (10 drops) from each component in 4-5 vials. These vials are cryopreserved at the same time as the component and stored overnight in liquid nitrogen. One vial is thawed in order to determine viability by growing 5,000 to 10,000 cells in a stemcell media for 2 weeks. Colonies or clusters of cells are observed and counted using a microscope. After the Stem Cells have been tested, they are approved by a transplant physician for transplant. The stem cells are stored until the patient has received high dose therapy.
Additional methods are available for processing according to the patients specific needs. Red blood cells can be depleted if the recipient and the donor have an ABO/Rh mismatch. Density gradient separation is a method to deplete red blood cells and granulocytes. Purification of stem cells using antibodies and immunomagnetic beads or affinity columns is a method that depletes all cells except for the true stem cells. The volume is considerable reduced to 5-10 mI compared to 50-100mL for cryopreservation.
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Therapy and Transplantation
Patients are admitted to an oncology/bone marrow transplant unit to receive 2 to 6 days of high dose chemotherapy with or without total body irradiation, based on the specific disease type and stage being treated. This therapy, called the conditioning or preparative regimen, is given to destroy malignant cells; to destroy the bone marrow to make room for the new cells; and to suppress the immune system so that it will not reject the new bone marrow.
The transplant will take place 36 to 72 hours after completion of therapy. It is not a surgical procedure, as is often thought, and occurs in the patient's room. The stem cells, whether harvested directly from the bone marrow or collected apheresis from the peripheral blood, are infused or transplanted into the patient intravenously after premedication is given.
Although processed prior to storage, the stem cells will still contain a small amount of red blood cells giving them a pink-colored appearance. During the transplant and for a few hours afterwards, patients are observed for fever, chills, allergic-type reactions, bradycardia, hypotension, nausea, vomiting, or diarrhea.
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Pancytopenia to Engraftment
The days following transplant are the most critical. The conditioning regimen will have destroyed the patient's bone marrow leaving the patient immunocompromised with severe pancytopenia: no white blood cells, decreased red blood cells and very few platelets. The infused stem cells are making their way to cavities of the large bones to begin engraftment. It will be 9 to 42 days until engraftment takes place and the production of the normal blood cells is seen. During this time, the patient is at risk for bleeding and is susceptible to infection. The patient may require transfusions of red blood cells and platelets until the bone marrow recovers.
Neutropenic precautions are the steps taken to minimize the patient's exposure to pathogens that may cause infection: No plants or cut flowers are allowed in the patient's room, his or her diet has no fresh fruits or vegetables during this time, the number of visitors and personnel entering the room is kept to a minimum, and the patient is asked to wear a mask when leaving the room. Patients are placed on prophylactic anti-infective agents: antivirals, antifungals, and antibiotics. Daily showers and frequent oral care are encouraged. The most important infection precaution observed is vigorous hand washing with an antibacterial soap for all who enter the patient's room.
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Other Potential Complications Related to Transplantation
The side effects of chemotherapy and radiation therapy can be more extreme in transplantation than in standard therapy because of the very high doses of therapy used. They may include:
nausea
vomiting
diarrhea
stomatitis
fatigue and malaise
anorexia and taste changes
amenorrhea and infertility
hair loss and changes in body image
The potential for irreversible and life-threatening damage to the heart, lungs, kidneys, liver and bladder is present. The possibility of developing secondary malignancies as a result of high-dose therapy is a complication that should be considered.
Graft versus host disease can occur in allogeneic transplantation when the lymphocytes of the donor stem cells, the graft, recognizes the patient's body, the host, as foreign. The graft will attack the organs of the skin, gastrointestinal tract, and liver predominantly.
Veno-occlusive disease is a complication in the liver that is caused by the effect of high doses of chemotherapy and radiation therapy on the blood vessels supplying the liver.
Failure to engraft, when the new bone marrow is unable to colonize and produce adequate cells, is a risk that varies with the type of transplant and patient-specific risk factors. It is a relatively rare occurrence and may require additional transplantation of cells without further chemotherapy.
There are many psychosocial considerations for the patient family undergoing transplantation. Facing a life-threatening illness and a life-threatening treatment regimen is very difficult in the first place. This can be complicated by financial and insurance concerns related to therapy, as well as prolonged physical discomfort and feelings of isolation. The psychosocial needs should be assessed and addressed as aggressively as any physiological problem.
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Articles on the specific applications of SCT
ABC of clinical haematology: Bone marrow and stem cell transplantation
Autologous Stem Cell Transplantation for Treatment of Autoimmune Diseases
Hepatitis Viruses and Hematopoietic Cell Transplantation: A Guide to Patient and Donor Management
Cord Blood Stem Cell Transplantation -- Facts Behind the Media Hype
Current Status of Treatment for Chronic Myelogenous Leukemia-
Autologous Stem Cell Transplantation (Section 9)
Epithelial Stem-Cell Transplantation for Severe Ocular-Surface Disease |
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